Guns, including pistols, revolvers, semiautomatics, rifles, and shotguns are used in the commission of thousands of crimes per day throughout the country. There is growing interest in tracing the source and movement of guns. Often crimes are solved by being able to associate one crime with another, or a crime with a particular gun, or a gun with a particular person. Tools used during commission of a crime often leave characteristic marks at the crime scene, which marks can be useful in linking together various crimes, or in linking a crime to a weapon, or in linking a crime to a criminal. The evidence that allows such links to be developed involves particular tool marks, which each weapon makes on the bullets and shell casings fired from it.
Because the evidence must be protected against contamination and the chain of custody must be preserved, and since it is often desirable to compare markings obtained at one crime scene to those obtained at others, it has become common to create images of the markings and use computer systems for automated comparison of the images from newly obtained specimens with databases of previously obtained images. Only after a possible match has been determined are the physical specimens viewed side by side.
Ammunition for handguns, rifles and shotguns all contain an outer casing or shell, a primer device to ignite, a fast burning material which is a form of nitrocellulose, and a projectile to fly out the barrel. Ammunition that travels through a particular gun receives scratches from the barrel of the gun, indentation from the firing pin, and perhaps other marks from the breech face and ejector. These tool marks are characteristic of the particular weapon used, and may be used to link a bullet or casing with the weapon from which they were fired.
The shell casing receives marking from the firing pin hitting the primer, from the back pressure of the gas expansion forcing the casing against the breech face of the firing pin housing which may have marks or defects which transfer onto the primer and/or casing. These marks may be a result of manufacturing defects, or hand finishing done in high quality weapons. Breech face marks can be compared just as can firing pin marks, either by firing a test round or by examining the weapon when the weapon is available, or by comparing corresponding marks on two bullets or casings which are suspected of coming from the same weapon. If the same weapon is used to fire the same type ammunition at the same type target from the same distance, comparable patterns will be produced on the bullet and casing. If the ammunition is changed, the patterns will be somewhat different.
Tool marks can be also transferred to the casing by the extractor and ejector of semiautomatics and automatics. Such marks include indentations and striations. The ejector is likely to gouge the casing. The extractor pulls the casing from the breech, imposing perhaps additional striations and indents before it hits the ejector.
Class characteristic marks vary with calibre, load, material used for the bullet or shot, bullet weight, its impact behavior, material used for the casing, and identification stamped into the bullet and casing. Intentional marks on ammunition include information stamped on the face of the bullet casing during molding of the shells. Some casings and bullets also have an indented ring or rings around the circumference called a canellure. These are smeared with grease or wax as sealer, making the bullet water-resistant and providing some lubrication as it is forced through the barrel. The canellures on the casing are imprinted within a quarter inch from the top after the bullet is inserted. This crimp acts to seal the round and hold the bullet in the casing. Canellures may contain imprint information unique to the manufacturer and perhaps to a particular crimping tool.
Ammunition for handguns, rifles and shotguns all contain an outer casing or shell, a primer device to ignite, a fast burning material which is a form of nitrocellulose, and a projectile to fly out the barrel. Ammunition that travels through a particular gun receives scratches from the barrel of the gun, indentation from the firing pin, and perhaps other marks from the breech face and ejector, in addition to fingerprints. The hand of the person firing the weapon receives a spray of gun residue which may be characteristic of a particular type of weapon and ammunition. The target receives some degree of blowback from the weapon and the target itself, plus perhaps fragments of any materials through which the bullet passed.
Markings on ballistic items which are of interest to forensics include:                Fingerprints on the weapon, projectile, or casing.        Other debris and material on the weapon, projectile, or casing.        Characteristics on the projectile or casing associated with the particular brand and type of ammunition used.        Characteristics on the projectile or casing associated with the particular weapon used. These are called tool marks.        
During manufacture, grooves are cut into the hard steel of pistol and rifle barrels, spiraling from the chamber to the muzzle. They cause the bullet to spin, which results in the bullet having a cleaner trajectory and the weapon having more accurate targeting. The raised areas between grooves are called lands. Bullets are intended to be fired in a particular calibre weapon. Nitrocellulose burns to produce an expanding gas, which drives the bullet through the barrel. The resulting heat causes the bullet to expand and softens its surface. Lead bullets are particularly prone to softening. The bullet is blown out of the shell casing and forced into the barrel, which is tighter. As a result, the lands are cut into the moving bullet and the surface is squeezed into the grooves. The high points of the barrel cause scratches in the bullet, which are referred to as striation evidence. The material used in the bullet determines the depth of the striations.
The number, width and depth of the grooves and the angle and direction (right or left) of their spiral are determined by the manufacturer. Lands and grooves together are called rifling. Rifling marks are transferred to the bullet as it is forced through the barrel. Some marks indicate the class of weapon used, while others indicate the particular gun or barrel used in the case of weapons with interchangeable barrels.
Most weapons other than Colt use a right twist. The number of lands or grooves typically is four to seven. The width varies depending on the number and the calibre. The degree of twist is measured by the distance traveled during one complete rotation of the bullet. A typical Colt handgun can be described as a left twist, six lands, 1/12. Most handgun barrels are shorter than 12 inches. Therefore, the energy transferred to spinning the bullet is not as great as in a rifle.
When the ammunition used is smaller than that designed to be used in a weapon, the result is loss of energy and penetration. In addition, since the bullet is loose within the barrel, inconsistent striations will occur on various bullets fired through the weapon.
Maintenance done on a weapon can alter the characteristics imposed on its ammunition. Use, cleaning, corrosion, and intentional damage to a barrel can all affect the ability to match ammunition used in it over time. Rust or corrosion will alter fine details. Some semiautomatics and automatics have interchangeable barrels. After exiting the barrel, the bullet may receive additional distortions as it passes through various materials. The result may be to destroy the forensic value of the bullet, or may be just to require further analysis.
The shell casing receives marking from the firing pin hitting the primer, from the back pressure of the gas expansion slamming the casing against the breech face of the firing pin housing which may have marks or defects which transfer onto the primer and/or casing. These marks may be a result of manufacturing defects, or result from the hand finishing done in high quality weapons. Breech face marks can be compared just as can firing pin marks, either by firing a test round or by examining the weapon when the weapon is available, or by comparing corresponding marks on two bullets or casings which are suspected of coming from the same weapon. If the same weapon is used to fire the same type ammunition at the same type target from the same distance, comparable patterns will be produced on the bullet and casing. If the ammunition is changed, the patterns will be different.
Shell casings are ejected immediately from automatic and semis, and so fall close to where the firing occurs. Revolvers retain the casings until intentionally ejected, and so are often carried away from the crime scene. Tool marks can be transferred to the casing by the extractor and ejector of semis and automatics. Such marks include indentations and striations. The ejector is likely to gouge the casing. The extractor pulls the casing from the breech, imposing perhaps additional striations and indents before it hits the ejector.
Class characteristic marks vary with calibre, load, material used for the bullet or shot, bullet weight, its impact behavior, material used for the casing, and identification stamped into the bullet and casing. Intentional marks on ammunition include information stamped on the face of the bullet casing during molding of the shells. Some casings and bullets, particularly lead, also have an indented ring or multiple rings around the circumference called a canellure. These are smeared with grease or wax when first inserted into the casing. The material acts as sealer, making the bullet water-resistant and providing some lubrication as it is forced through the barrel. The canellures on the casing are imprinted within a quarter inch from the top after the bullet is inserted. This crimp acts to further seal the round and hold the bullet in the casing. Canellures may contain imprint information unique to the manufacturer and perhaps even to a particular crimping tool.
Types of ammunition are distinguished by their size. All handguns are designated by calibre in either inches or millimeters. For example a .22 calibre is 22/100 inch. The length of the shell varies to increase the capacity of gunpowder. A 0.22 may be short, long or long rifle, plus a magnum load. All magnum loads contain more gunpowder and may propel a heavier bullet. Magnum weapons are always designed to be heavier to contain the increased force. A magnum weapon can chamber a regular load, but a regular weapon cannot chamber a magnum round.
Some 0.22 handguns and rifles can chamber only the short, some can chamber short and long, and others can handle all three. The 0.22 short and long have the same weight bullet, while the long has more powder. The long rifle has more powder still and a heavier bullet but still is not considered a magnum load. All .22 rounds of these types are rim fire primers, rather than center core primers. Some high-powered rifles can proper .22 calibre bullets with center core primers, but the shell casings are huge relative to this group.
The .25 calibre pistol purse gun is designed to be easily concealed. There are .32 calibre revolvers and semiautomatics. The .32 calibre rim fire primers were used in early revolvers, but all modern rounds are center core primers.
There are two types of ammunition for .38 calibre (.357) revolvers; the .38 S&W and the .38 Special. The .38 Smith and Wesson is a shorter shell with smaller load. It has been superceded by the more powerful .38 Special, designed for law enforcement use. The maximum stopping power for the .38 calibre became available when the .357 magnum was marketed. The weapon fires a heavier bullet, with longer casing that contains additional gunpowder. However, when firing a solid lead bullet, the weapon proved a hazard to unintended secondary targets, because the penetrating power was too great.
For police use, a combination of stopping power and safety are desired. Several .38 calibre bullets are designed to impact only the first target, transferring all the kinetic energy to it. Hollow point bullets were designed for this purpose. A hollow point is a soft lead bullet encased in a thin steel jacket. The point is actually an opening with thin lead walls. On impact the wall flare out. The final shape of the bullet, in the side view is similar to a mushroom. Hence the term “mushrooming” is applied. Hollow points don't necessarily mushroom every time, and can still harm secondary targets, such as by passing through soft tissue of the target and injuring someone behind.
Glasser bullets were developed with a prefragmented round constructed of a thin metal skin packed with very small lead particles suspended in liquid Teflon. The bullet's weight is equal to that of a solid lead bullet, resulting in equivalent kinetic energy. When the Glasser hits any resisting surface, the thin walls flatten out, distributing the tiny lead particles over a wide area. A bullet hitting a human or animal penetrates and opens up, transferring all its energy to the inside of the target, having an elephant gun effect. The Glasser offered improved safety since all energy was expended when the bullet hit something. Errant shots striking pavement would not ricochet. Bullets hitting a house would not penetrate walls. Other manufacturers also offer prefragmented bullets.
Shotgun shells are different from handgun and rifle ammunition. Although solid metal casings have been used, they are now mostly replaced by a combination casing with a metal face and base connected to a plastic or waxed cardboard cylinder. Shotgun shells are available in gauges 10, 12, 14, 16, 20, and 410. The cylinder is crimped shut, sealing in the projectiles, shot and other materials with the gunpowder. The face contains manufacturer's information and markings and houses a center core primer. The actual projectiles vary. The shot, which was traditionally lead, is gradually being replaced by steel balls. Lead shot has been cited by environmentalists as causing lead contamination in waterways.